1. Field of the Invention
Methods and layers consistent with what is disclosed herein relate to a fabrication method for a quantum dot sensitized solar cell using supercritical fluid or subcritical fluid and a quantum dot sensitized solar cell prepared thereby.
2. Description of the Related Art
In recent years, due to problems such as energy depletion, environmental pollution and global warming associated with the excessive use of fossil fuels, development and production of renewable energy has been urgently required. Solar cell, which produces electric energy by using solar light, has been widely studied and developed as a next generation energy resource. This is mainly because solar cell does not contribute to the global warming, and solar cell only requires solar energy as a source which is limitless. Currently, various types of solar cells, such as crystalline silicon (c-Si) based-solar cell, amorphous silicon (a-Si) solar cell, thin film solar cell based on copper indium gallium diselenide (CIGS) and cadmium telluride (CdTe), polymer solar cell, and dye-sensitized solar cell, have been developed and some types of solar cell including crystalline silicon (c-Si) based-solar cell are commercially available. Among these, dye-sensitized solar cell receives a considerable attention as a next generation solar cell, mainly because it costs much lower than crystalline silicon (c-Si)-based solar cell, and provides relatively high energy conversion efficiency (˜11%).
For example, U.S. Pat. No. 4,927,721 discloses a dye-sensitized solar cell, in which dye attached to nanocrystalline semiconductor network of mesoporous or nano-structure, often characterized by its high surface area. The electrons generated from the light-excited dye are transmitted to the nanocrystalline semiconductor network, and it transfers the electrons to the working electrode. The crucial part to determine energy conversion efficiency and cell stability against photodegradation is the dye itself. It is well known that only a limited number of dyes are effective to give high quantum yields. Typically dyes cannot use a wide range of wavelengths of the light reaching the earth. Dyes become unstable when exposed to solar light for an extended period of time due to the photodegradation.
Accordingly, a quantum dot sensitized solar cell, which uses a wider range of light wavelengths and is stable to solar light, has been developed (Mora-Seró, I.; Giménez, S.; Fabregat-Santiago, F.; Gómez, R.; Shen, Q.; Toyoda, T.; Bisquert, J. Acc. Chem. Res. 2009, 42, 1848-1857).
Quantum dot has the characteristics of a semiconductor, and is made from at least one compound selected from the group consisting of: a compound consisting of the first element selected from groups 2, 12, 13, and 14 and the second element selected from group 16 of the periodic table; a compound consisting of the first element selected from group 13 and the second element selected from group 15 of the periodic table; and a compound consisting of an element selected from group 14 of the periodic table, which may include, but not limited to, CdS, CdSe, CdTe, CdO, PbS, PbSe, PbTe, ZnS, ZnSe, ZnTe, GaAs, GaN, GaP, GaSb, InP, InSb, AlAs, or AlSb. The quantum dot sensitized solar cell is composed of a material that is capable of absorbing lights of different wavelengths according to the sizes of the quantum dots, and forming electrons and electron holes, and is thus capable of utilizing a light with wider range of wavelengths by adjusting the type or the size of the quantum dot. Furthermore, being composed of inorganic material, the quantum dot sensitized solar cell is capable of providing excellent stability toward the photodegradation. Furthermore, compared to the conventional dye-sensitized solar cell, the quantum dot sensitized solar cell can provide multiple exciton generation (MEG) effect and thus can produce more electrons. Thus, theoretical power conversion efficiency is far greater than that of dye-sensitized solar cell or silicon-based solar cell.
A quantum dot sensitized solar cell is generally fabricated by forming particles including nanoparticles and nanorods of metal oxide and metal oxides such as TiO2, ZnO, SnO2, or WO3 in nano-meter level into a film-like structure with mesoporous or nano-structured morphology, and adsorbing or coating quantum dots onto a surface and interior parts of the metal oxide film-type transparent electrode of electron acceptor, to which the quantum dots are adhered in the size ranging from several nanometers to several tens of nanometers to absorb a wide range of wavelengths and thus increase efficiency of the solar cell. Conventionally, the quantum dots are adsorbed onto the metal-oxide electron acceptor transparent electrode with mesoporous or nano-structure by the method of: dispersing pre-synthesized quantum dots in an organic solvent and introducing a transparent electrode in the organic solvent in which the quantum dots are dispersed; attaching the quantum dots onto the surface of the metal oxide that forms the transparent electrode using appropriate organic linker molecules; or absorbing the first element that constitutes the quantum dot dissolved in an appropriate solvent and then introducing a solution or organic solvent fluid containing the second element. However, the above-mentioned methods have drawbacks. That is, since water or organic solvents used in the above-mentioned methods have a relatively high viscosity, the pre-synthesized quantum dots, and the precursor of the quantum dot are transmitted to the inner pores of the metal oxides of mesoporous or nano-structure at a relatively slow rate, and since water or organic solvents have high surface tension, it is difficult for the quantum dot to be penetrated into the metal oxide transparent electrode of mesoporous or nano-structure, and dispersed and adsorbed into the pores with good uniformity and regularity. Therefore, the adsorption of the quantum dot is more difficult in the interior regions of the metal oxide film with mesoporous or nano-structured morphology (Chang, C. H.; Lee, Y. L. Appl. Phys. Lett. 2007, 91, 053503/1-053503/3; Lee, H.; Wang, M.; Chen, P.; Gamelin, d. R.; Zakeeruddin, S. M.; Gräitzel, M.; Nazeeruddin, M. K. Nano Lett. 2009, 9, 4221-4227).
Accordingly, the inventors of the present application have studied on the quantum dot sensitized solar cell and developed and completed a method of adsorbing a quantum dot onto a conductive thin layer substrate coated with a metal oxide film having mesoporous or nano-structure using supercritical fluid or subcritical fluid, and a quantum dot sensitized solar cell using the same.